“The main conclusion,” Dr. Murthy continued, “is that children are infected at rates that may be comparable to adults, with severity that’s much less, but that even within the kids, there’s a spectrum of illness and there’s a handful that require more aggressive therapy.”
More than 60 percent of the 125 children who became severely ill or critically ill were age 5 or younger, the study reported. Forty of those were infants, under 12 months old.
Dr. Tong said he believed that younger children were more susceptible to infection because their respiratory systems and other body functions are rapidly developing.
Dr. Andrea Cruz, an associate professor of pediatrics of Baylor College of Medicine and co-author of a commentary about the study, said that preschoolers and babies likely get sicker because of their “immune system immaturity.”
“They haven’t been exposed to viruses before and therefore they can’t mount an effective immune response,” she said in an interview.
Scientists are actively trying to determine why so many children appear to emerge relatively unscathed by the new coronavirus, a pattern that also characterized the earlier outbreaks of the closely-related SARS virus in China and MERS in the Middle East. Cases of children with the new coronavirus infection in Italy, Singapore and South Korea seem to be similar, Dr. Murthy said.
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A theory that is gaining increasing currency is that the receptor or protein in human cells that the viral particles bind to, called the ACE2 receptor, is not expressed as prominently in young children or might be a different shape, Dr. Murthy said.
“It might not be as developed in children as in adults,” he said, which might make it tougher for the spikes on the tiny viral particles to bind and gain entry to the cells so the virus can replicate.
Another theory is that “most kids have healthier lungs” than adults, Dr. Cruz said. Adults have likely been more exposed to pollution over their lifetime and adults with severe coronavirus disease have tended to have underlying health conditions or weakened or aging immune systems.
It’s also possible, experts say, that children’s immune systems don’t rev up to attack the virus as much as adult immune systems do. Doctors have found that some of the serious damage infected adults have endured has been caused not just by the virus itself, but by an aggressive immune response that creates destructive inflammation in the body’s organs.
The new study, while large and included cases across China, not just where the outbreak originated in Wuhan, leaves many unanswered questions. For example, the researchers found that more of the severe and critical cases were in children with suspected — instead of confirmed — coronavirus infection, raising the possibility that other infections wreaked havoc on their bodies, in addition to or possibly even instead of Covid-19.
It’s also unclear whether the United States can expect the relatively small numbers of child cases reported in China or should brace for more.
“The age pyramid in China is really different than the U.S. — they have a lot fewer kids than we do,” said Dr. Cruz, who believes, as other experts do, that large numbers of people with mild or asymptomatic disease have not been recorded because testing was not done in those cases. “You’ve had a lot of under-testing in children because the focus has been on adults. It’s likely we’ve been underestimating the disease burden in kids.”
Answering questions about coronavirus in children could reverberate well beyond the pediatric population. It could shed light why some patients are most at risk. And, said Dr. Murthy, studying the physiology of those who are less affected could help in the development of treatment and a vaccine.
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